In vivo analysis of cerebellar circuitry has focused almost exclusively on Purkinje cells, identified by their iconic patterns of complex and simple spikes (CSs and SSs). However, views of cerebellar function based exclusively on the physiology of Purkinje cells ignore the role of interneurons and distort the attributes of cerebellar afferent systems. It is universally assumed that SSs are modulated by the activity of the mossy fiber-granule cell-parallel fiber projection to Purkinje cell dendrites. In fact, during natural vestibular stimulation, vestibular primary afferent mossy fiber afferents discharge out of phase with the SSs recorded from nodular Purkinje cells. Consequently, it is unlikely that the cerebellar output signal merely reflects a gain-controlled version of the mossy fiber input signal. We proposed that SS modulation reflects the action of climbing fibers on cerebellar interneurons. We will test specific versions of this hypothesis by recording from identified interneurons. We have three objectives. First, we will record extracellularly from interneurons in the uvula-nodulus of anesthetized mice during natural vestibular stimulation. Interneurons will be labeled juxtacellularly with neurobiotin. The depth and phase of modulation of interneuronal discharge relative to that of Purkinje cell CSs and SSs will indicate which interneurons could modulate SSs. Second, we will study how the modulated activity of interneurons and Purkinje cells is altered by a unilateral labyrinthectomy (UL). Following a UL, the ipsilateral uvula-nodulus is accessible to vestibular information mediated only by climbing fibers whose modulation depends on the contralateral, intact labyrinth. Third, we will also make microlesions in the p-nucleus and dorsomedial cell column (dmcc) in the contralateral inferior olive. This will leave one side of the cerebellum accessible to vestibular information mediated only by vestibular mossy fibers. We will compare the effects of reduced vestibular signaling on interneurons and Purkinje cells. Fourth, we will microinject miRNAs in viral vectors with a cell specific promoter to selectively reduce expression of GABA-A alpha 1 receptors in nodular Purkinje cells. Fifth, we will also use miRNAs to selectively reduce synthesis of GABA in Golgi cells. We will analyze the effects of "knocking down" GABAergic signaling in these two cell types. We will characterize the stimulus-modulated functions of identified interneurons for the first time. We will interfere with cerebellar circuitry at a cellular level and test the role of interneurons in the modulation of SSs. The proposed research will speed application of molecular techniques to the treatment of patients with cerebellar disorders.